Alite nucleation

The initial oversaturation of the melt towards ahte is low and principally the heterogeneous nucleation takes place, primarily as a result of the decrease of Gibbs free energy on phases boundary [42]. In this initial period alite nucleation can occiu... 

Calorimetric studies show that pfa retards the reaction of alite in the early stage of reaction (G70,J30,H51,W29), but with one exception (W29) studies on the middle stage show the alite reaction to be accelerated (K45,L47,H51,T44,D12). SEM shows that C-S-H and CH are deposited on the surfaces of pfa grains before these have started to react significantly (D28). The accelerating effect is probably due mainly or entirely to the provision of additional nucleation sites on the pfa, and occurs with other fine powders (K46). A QXDA study showed that the aluminate and ferrite react more rapidly in the presence of pfa, but with belite there was no detectable effect up to 28 days and a marked reduction in rate of consumption thereafter (D12). 

Increase in alite content, decrease in belite and free lime Increased nucleation of alite by adding 0.5% C3S (less than 20 pm) to raw mix (Strunge, Kndfel, and Dreizier, 1985)... 

Hydration of fly ash cement differs from pure cement in terms of the hydration rates of the clinker phases, amount of calcium hydroxide formed, composition of the clinker hydration products, and additional hydration products from the reaction of the fly ash.I l Lower amounts of lime are formed in the presence of fly ash because ofthe pozzolanic reaction between the fly ash and lime formed in cement hydration. Fly ash generally retards the reaction of alite in the early stages and accelerates the middle stage reaction. The accelerated reaction is attributed to the existence of nucleation sites on fly ash particles. The aluminate and ferrite phases hydrate more rapidly in the presence of fly ash, and also there is a significant difference in the hydration rate of the belite phase up to 28 days. Table 1 gives the relative hydration rates of cement compounds in the presence of fly ash as derived from conduction calorimetry. [" 1 It can be seen that the earlier rates of hydration are generally retarded, and the later stage hydration rates are accelerated. 

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